Geant4手册目录
Geant4: 2.4.  How to Specify Particles

2.4.  How to Specify Particles

G4VUserPhysicsList is one of the mandatory user base classes described in Section 2.1. Within this class all particles and physics processes to be used in your simulation must be defined. The range cut-off parameter should also be defined in this class.

The user must create a class derived from G4VuserPhysicsList and implement the following pure virtual methods:

ConstructParticle();      // construction of particles
ConstructProcess();       // construct processes and register them to particles
The user may also want to override the default implementation of the following virtual method:
SetCuts();                // setting a range cut value for all particles

This section provides some simple examples of the ConstructParticle() and SetCuts() methods. For information on ConstructProcess() methods, please see Section 2.5.

2.4.1.  Particle Definition

Geant4 provides various types of particles for use in simulations:

  • ordinary particles, such as electrons, protons, and gammas
  • resonant particles with very short lifetimes, such as vector mesons and delta baryons
  • nuclei, such as deuteron, alpha, and heavy ions (including hyper-nuclei)
  • quarks, di-quarks, and gluon

Each particle is represented by its own class, which is derived from G4ParticleDefinition. (Exception: G4Ions represents all heavy nuclei. Please see Section 5.3.) Particles are organized into six major categories:

  • lepton,
  • meson,
  • baryon,
  • boson,
  • shortlived and
  • ion,

each of which is defined in a corresponding sub-directory under geant4/source/particles. There is also a corresponding granular library for each particle category.

2.4.1.1.  The G4ParticleDefinition Class

G4ParticleDefinition has properties which characterize individual particles, such as, name, mass, charge, spin, and so on. Most of these properties are "read-only" and can not be changed directly. G4ParticlePropertyTable is used to retrieve (load) particle property of G4ParticleDefinition into (from) G4ParticlePropertyData.

2.4.1.2.  How to Access a Particle

Each particle class type represents an individual particle type, and each class has a single object. This object can be accessed by using the static method of each class. There are some exceptions to this rule; please see Section 5.3 for details.

For example, the class G4Electron represents the electron and the member G4Electron::theInstance points its only object. The pointer to this object is available through the static methods G4Electron::ElectronDefinition(). G4Electron::Definition().

More than 100 types of particles are provided by default, to be used in various physics processes. In normal applications, users will not need to define their own particles.

The unique object for each particle class is created when its static method to get the pointer is called at the first time. Because particles are dynamic objects and should be instantiated before initialization of physics processes, you must explicitly invoke static methods of all particle classes required by your program at the initialization step. (NOTE: The particle object was static and created automatically before 8.0 release)

2.4.1.3. Dictionary of Particles

The G4ParticleTable class is provided as a dictionary of particles. Various utility methods are provided, such as:

FindParticle(G4String name);         // find the particle by name
FindParticle(G4int PDGencoding)      // find the particle by PDG encoding .

G4ParticleTable is defined as a singleton object, and the static method G4ParticleTable::GetParticleTable() provides its pointer.

As for heavy ions (including hyper-nuclei), objects are created dynamically by requests from users and processes. The G4ParticleTable class provides methods to create ions, such as:

G4ParticleDefinition* GetIon(  G4int    atomicNumber,  
                               G4int    atomicMass, 
                               G4double   excitationEnergy);

Particles are registered automatically during construction. The user has no control over particle registration.

2.4.1.4.  Constructing Particles

ConstructParticle() is a pure virtual method, in which the static member functions for all the particles you require should be called. This ensures that objects of these particles are created.

WARNING: You must define "All PARTICLE TYPES" which are used in your application, except for heavy ions. "All PARTICLE TYPES" means not only primary particles, but also all other particles which may appear as secondaries generated by physics processes you use. Beginning with Geant4 version 8.0, you should keep this rule strictly because all particle definitions are revised to "non-static" objects.

For example, suppose you need a proton and a geantino, which is a virtual particle used for simulation and which does not interact with materials. The ConstructParticle() method is implemented as below:

Example 2.14.  Construct a proton and a geantino.

 void MyPhysicsList::ConstructParticle()
 {
   G4Proton::ProtonDefinition();
   G4Geantino::GeantinoDefinition();
 }



Due to the large number of pre-defined particles in Geant4, it is cumbersome to list all the particles by this method. If you want all the particles in a Geant4 particle category, there are six utility classes, corresponding to each of the particle categories, which perform this function:

  • G4BosonConstructor
  • G4LeptonConstructor
  • G4MesonConstructor
  • G4BarionConstructor
  • G4IonConstructor
  • G4ShortlivedConstructor.

An example of this is shown in ExN05PhysicsList, listed below.

Example 2.15.  Construct all leptons.

void ExN05PhysicsList::ConstructLeptons()
{
  // Construct all leptons
  G4LeptonConstructor pConstructor;
  pConstructor.ConstructParticle();
}



2.4.2.  Range Cuts

To avoid infrared divergence, some electromagnetic processes require a threshold below which no secondary will be generated. Because of this requirement, gammas, electrons and positrons require production threshold. This threshold should be defined as a distance, or range cut-off, which is internally converted to an energy for individual materials. The range threshold should be defined in the initialization phase using the SetCuts() method of G4VUserPhysicsList. Section 5.5 discusses threshold and tracking cuts in detail.

2.4.2.1.  Setting the cuts

Production threshold values should be defined in SetCuts() which is a virtual method of the G4VUserPhysicsList. Construction of particles, materials, and processes should precede the invocation of SetCuts(). G4RunManager takes care of this sequence in usual applications.

This range cut value is converted threshold energies for each material and for each particle type (i.e. electron, positron and gamma) so that the particle with threshold energy stops (or is absorbed) after traveling the range cut distance. In addition, from the 9.3 release ,this range cut value is applied to the proton as production thresholds of nuclei for hadron elastic processes. In this case, the range cut value does not means the distance of traveling. Threshold energies are calculated by a simple formula from the cut in range.

Note that the upper limit of the threshold energy is defined as 10 GeV. If you want to set higher threshold energy, you can change the limit by using "/cuts/setMaxCutEnergy" command before setting the range cut.

The idea of a "unique cut value in range" is one of the important features of Geant4 and is used to handle cut values in a coherent manner. For most applications, users need to determine only one cut value in range, and apply this value to gammas, electrons and positrons alike. (and proton too)

The default implemetation of SetCuts() method provides a defaultCutValue member as the unique range cut-off value for all particle types. The defaultCutValue is set to 1.0 mm by default. User can change this value by SetDefaultCutValue() The "/run/setCut" command may be used to change the default cut value interactively.

WARNING: DO NOT change cut values inside the event loop. Cut values may however be changed between runs.

It is possible to set different range cut values for gammas, electrons and positrons by using SetCutValue() methods (or using "/run/setCutForAGivenParticle" command). However, user must be careful with physics outputs because Geant4 processes (especially energy loss) are designed to conform to the "unique cut value in range" scheme.

Beginning with Geant4 version 5.1, it is now possible to set production thresholds for each geometrical region. This new functionality is described in Section 5.5.

<-- 2.3. How to Specify Materials in the Detector  2.5. How to Specify Physics Processes --> "